Single-unit magnetic coupler and switching power supply

ABSTRACT

The invention relates to a unitary magnetic coupler including a first inductor (Lp) consisting of a first winding of phase φ and having a number N of turns between the two ends of the first winding and, magnetically coupled to the first inductor (Lp), a second inductor (Ls) consisting of a second winding of the same phase φ and having the same number N of turns between the two ends of the second winding, where the ends of the first and second windings of the unitary magnetic coupler are interconnected using links consisting of capacitors (C 1,  C 2 ) of equal value.

TECHNICAL FIELD

The invention relates to a unitary magnetic coupler.

Other subjects of the invention are a switch mode power supply and datatransmission equipment employing such a coupler.

The field of the invention is that of power supplies designed to deliverdirect-current from an alternating-current (AC) or direct-current (DC)power distribution network.

BACKGROUND OF THE INVENTION

Specifically, power supplies operating at low power levels, typicallyless than 150 W, will be considered.

One aim is to minimize the size and weight of the power supplies.

“Flyback” or “forward” power supplies are low- power switch mode powersupplies employed frequently, particularly because they are simple tocontrol. The flyback design is a very interesting case because of itsreduced size arising from the fact that it needs only one magneticelement to achieve the power conversion.

It will be recalled that in switch mode power supplies the DC voltage ischopped by a switch that is switching on and off at a frequency calledthe switching frequency.

A flyback power supply configuration and a forward power supplyconfiguration will now be described; these are examples chosen fromvarious known configurations.

A flyback power supply, a circuit diagram of which is shown in FIG. 1a), is an energy storage switch mode power supply.

It comprises a primary circuit P consisting of, in series, a voltagesource V_(in), a switch M, for example a MOS transistor and an inductorLp made up of a winding of Np turns, and a secondary circuit Sconsisting of, in series, an inductor Ls made up of a winding of Nsturns, magnetically coupled to Lp, a capacitor C_(out) connected to aload represented here by a resistor R_(load) and a rectifier D, forexample a diode.

For each of the windings of Lp and Ls, the phase φ, corresponding to thedirection of the winding, is identified by a circle. In the exampleshown, the first and second windings have the same phase.

The coupling circuit consisting of the primary inductor Lp and thesecondary inductor Ls is denoted by the transformer T.

The current flowing through the primary circuit is i_(p), and thevoltages across the terminals of the primary circuit and across theswitch are V_(in) and V_(M) respectively. The current flowing throughthe secondary circuit i_(s) is, and the voltages across the terminals ofthe secondary circuit and across the diode are V_(out) and V_(D)respectively.

In this “flyback” power supply design, current does not flow throughboth windings simultaneously. The operation of this power supply, calledan “inductive storage” supply, is based on energy transfer cycles madeup of a magnetic energy storage phase in the inductive element of theprimary circuit (in this case Lp), followed by a phase for transferringthis stored energy to a secondary source via the secondary circuit.

The various operating phases of this power supply will now be described,with reference to FIGS. 1 b) and 1 c).

Let us first recall a basic principle that underlies some of theexplanations to follow: it is impossible to force a voltagediscontinuity across the terminals of a capacitor and a currentdiscontinuity in an inductor.

When the switch M is closed (FIG. 1 b), i.e. during T_(on), the energyis stored in the inductor Lp; the diode does not conduct since thevoltage V_(D) across its terminals is negative and therefore the currenti_(s) is zero.

When the switch M is open, i.e. during T_(swt)-T_(on), where T_(swt) isthe switching period, the current i_(p) is zero (FIG. 1 c). Thecontinuity of the magnetic energy leads to the transfer of the energystored previously in the inductor Lp to the inductor Ls and also resultsin the diode D switching to its conducting state: D demagnetizes thetransformer T. This phase ends if the current in the diode D falls tozero or if the end of the switching period is reached.

FIGS. 1 d) and 1 e) show the waveforms in continuous mode, in which thecurrent i_(s) does not fall to zero at the end of the conducting phaseof the secondary-circuit diode D. To simplify the description, it isassumed that the current i_(p) changes instantaneously from its maximumvalue to zero.

The voltage V_(Lp) across the terminals of the inductor Lp, representedin FIG. 1 d), varies as a function of time between a maximum value ofV_(in) and a minimum value of −V_(out)×N_(p)/N_(s).

The current i_(p), represented in FIG. 1 e), varies as a function oftime between a maximum value of i_(Max) and zero; the current i_(s)varies as a function of time between zero and a maximum value ofi_(Max)×N_(s)/N_(p).

A forward power supply, a circuit diagram of which is shown in FIG. 2a), is a switch mode power supply that directly transfers energy.

It comprises a primary circuit P consisting of, in series, a voltagesource V_(in), a switch M, for example a MOS transistor and an inductorLp made up of a winding of N_(p) turns, and, in parallel with theinductor Lp and the switch M, a demagnetizing circuit for demagnetizingthe transformer which circuit may be a diode D_(dem) placed in serieswith an inductor L_(dem), magnetically coupled to Lp, made up of awinding of N_(dem) turns. The diode D_(dem) and the inductor L_(dem) maybe replaced by other components.

The secondary circuit S consists of, in series, an inductor Ls made upof a winding of Ns turns, magnetically coupled to Lp, a capacitorC_(out) connected to a load represented here by a resistor R_(load), aninductor L, a first rectifier D1, for example a diode, and, in parallelwith the inductor Ls and the rectifier D1, a second rectifier D2 whichmay also be a diode.

The phase φ of each of the windings of Lp, L_(dem) and Ls is identifiedby a circle. In the example given, the windings of Lp and Ls have thesame phase, opposite to that of the winding of L_(dem).

As in the previous case, the coupling circuit consisting of the primaryinductor Lp, the secondary inductor Ls and the inductor L_(dem) isdenoted by the transformer T.

The current flowing through the primary circuit is i_(p), and thevoltages across the terminals of the primary circuit and across theswitch are V_(in) and V_(M) respectively. The current flowing throughthe secondary circuit is i_(s), and the voltages across the terminals ofthe secondary circuit and across the diode D1 are V_(out) and V_(D1)respectively.

In this “forward” power supply design, both windings operatesimultaneously; there is a direct transfer of energy between theinductors Lp and Ls.

The various operating phases of this power supply will now be described,with reference to FIGS. 2 b), 2 c) and 2 d).

When the switch M is closed (FIG. 2 b), i.e. during T_(on), some of theenergy is stored in the inductor Lp (this energy is a “parasitic”quantity and therefore much less than the energy of the direct transfer)and the remaining energy is directly transferred between the inductorsLp and Ls, and the diode D1 conducts; a current i_(s) flows in thesecondary circuit; the diodes D2 and D_(dem) become nonconducting sincethe voltages across their terminals are negative.

When the switch M is open (FIG. 2 c), the diode D1 becomesnonconducting, and the diodes D2 and D_(dem) switch to the conductingstate. In accordance with the basic principle stated earlier, the diodeD2, called a freewheeling diode, provides continuity of the current isin the inductor L and the diode D_(dem) provides continuity of themagnetic energy stored in the inductor Lp during the previous phase(i.e. during T_(on)) by transferring this stored energy to V_(in) over atime given by T_(on)×N_(dem)/N_(p): D_(dem) demagnetizes the transformerT.

At the end of the demagnetizing phase (FIG. 2 d), i.e. duringT_(swt)−T_(on)×(1+N_(dem)/N_(p)) D_(dem) becomes nonconducting; D2remains conducting. This is the freewheeling phase.

FIGS. 2 e) and 2 f) show the waveforms. To simplify the description, itis assumed that the current i_(p) changes instantaneously from itsmaximum value to zero.

The voltage V_(Lp) across the terminals of the inductor Lp, representedin FIG. 2 e), varies as a function of time between V_(in) and−V_(in)×N_(p)/N_(dem).

The current i_(p), represented in FIG. 2 f), varies as a function oftime between a maximum value of i_(Maxp) and zero; the current is variesas a function of time between a maximum value of i_(Maxs) and a minimumvalue of i_(min).

From now on, it will be generally assumed that a primary circuit Pincludes at least one switch M placed in series with a voltage sourceV_(in) and a first inductor Lp, that a secondary circuit includes atleast one rectifier D placed in series with a second inductor Ls and acapacitor C_(out) connected to a load, and that the primary andsecondary circuits are coupled by a coupling circuit including at leastthe primary inductor Lp and the secondary inductor Ls magneticallycoupled to each other.

One aim is to further reduce the size and weight of these powersupplies.

In order to be able to use small components while achieving the sameenergy conversion possibilities in terms of the power available at theoutput, the switching frequency must be increased. This has thedrawbacks of increasing losses in the transformer and switch-relatedlosses in the other components, which in turn reduces the overallefficiency and therefore raises the temperature and reduces reliability.

High-frequency imperfections in the transformer are conventionallymodeled by a leakage inductance Lf in series with the inductor Lp, asshown in FIG. 3 a) for a flyback power supply and in FIG. 3 b) for aforward power supply.

In the case of a flyback power supply operating in discontinuous mode,in which the current is falls to zero at the end of the conducting phaseof the secondary-circuit diode D, the voltage across the terminals ofthe switch M when it opens can be given approximately by the followingformula:$V_{M} = {V_{in} + {\frac{Np}{Ns} \times V_{out}} + {L_{f} \times \frac{\mathbb{d}}{\mathbb{d}t}I_{p}}}$

When the switch opens, it is assumed that the current decreases linearlyfrom its maximum value to zero over a time Tfall which is theclosed/open switching time of the switch. Therefore, upon opening of theswitch M, and with Ton being the time over which the switch M is closed:$V_{M} = {V_{in} + {\frac{Np}{Ns} \times V_{out}} + {\frac{L_{f}}{L_{p}} \times V_{in} \times \frac{T_{on}}{T_{fall}}}}$

Hence, the leakage inductance results in a term representing anovervoltage across the terminals of the switch, in the form:${\frac{L_{f}}{L_{p}} \times V_{in} \times \frac{T_{on}}{T_{fall}}},$and the power Pf due to the leakage inductance is:${P_{f} = {\frac{1}{2} \times \frac{V_{in}^{2} \times T_{on}^{2}}{L_{p}^{2}} \times \frac{1}{T_{swt}} \times L_{f}}},$where T_(swt) is the switching period.

The energy stored in the leakage inductance is in general dissipatedduring the switching phases.

Furthermore, the switching-related losses upon opening of the switch areproportional to Tfall.

Therefore, reducing the switching time Tfall reduces theswitching-related losses but increases the term representing theovervoltage across the terminals of the switch.

For example, an opening time Tfall 100 times lower than the closure timeTon, and a leakage inductance Lf of about 1% of Lp, results in anovervoltage upon the switching action equal to the power supply voltageVin. The consequences of this would be disastrous as regards the voltagedimensioning of the switch M, in this case the transistor M which mustbe a high voltage range transistor and therefore more expensive and lesseffective.

In the case of a forward power supply, other equations are derived butthe same observations are made on interpreting them.

There are several types of circuits for countering the effect of theleakage inductance.

Dissipative RCD (i.e. Resistor, Capacitor, Diode) circuits are veryeffective in limiting overvoltages but they dissipate all the energystored in the leakage inductance resulting in a reduction in overallefficiency.

FIG. 4 shows an example of a flyback power supply employing an RCDcircuit. The capacitor C limits the term representing the overvoltageupon opening of the switch M; the resistor R discharges the voltageacross the terminals of C and thus dissipates the energy stored in theleakage inductance.

Snubber circuits are often employed to reduce the overvoltages acrossthe terminals of the switch M.

FIG. 5 shows an example of a flyback power supply employing a snubbercircuit that dissipates very little energy. As in the previous case, thecapacitor limits the overvoltages across the terminals of the switch M.To recover the energy stored in C, an oscillating circuit based on L andC inverts the voltage across the terminals of C. In practice, losses indiodes D1 and D2 and in the inductor L limit the portion of energyrecovered by the circuit. Furthermore, the oscillations of the LCcircuit must be damped, which also reduces the efficiency.

Lastly, such a circuit is more complex and therefore less reliable, andthe efficiency of the power supply would have improved only slightly.

SUMMARY OF THE INVENTION

One important aim of the invention is therefore to propose a circuit forreducing, in flyback or forward power supplies, overvoltages across theterminals of the switch M, switching-related losses and losses of energystored in the leakage inductance.

To achieve these aims, the invention proposes a unitary magnetic couplerincluding a first inductor Lp consisting of a first winding of phase φand having a number N of turns between the two ends of the first windingand, magnetically coupled to the first inductor Lp, a second inductor Lsconsisting of a second winding of the same phase φ and having the samenumber N of turns between the two ends of the second winding, whichunitary magnetic coupler is characterized in that the ends of the firstand second windings are interconnected using links consisting ofcapacitors of equal value.

This type of coupler, in which the inductor of the primary circuit hasthe same number of turns as the inductor of the secondary circuit,enables the same voltage to exist across the terminals of the primaryand secondary windings of the same phase and therefore a capacitive linkcan be used to counter the effect of the leakage inductance withoutincreasing switching-related losses.

Another subject of the invention is a switch mode power supply having aprimary circuit P coupled to a secondary circuit S by means of amagnetic coupling circuit, characterized in that the magnetic couplingcircuit is a unitary magnetic coupler as described above.

The power supply may be a flyback type or forward type.

As a preference, the primary circuit P and the secondary circuit S areable to generate at the terminals of each capacitor of the unitarymagnetic coupler a voltage that does not change as a function of theswitching frequency.

The invention also relates to data transmission equipment including atleast one data transmit-receive device connected to a two-wire data bus,characterized in that it includes a unitary coupler as described above,able to connect the data transmit-receive device to the two-wire databus.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent onreading the following detailed description, given by way of nonlimitingexample and with reference to the accompanying drawings in which:

FIGS. 1 a) to 1 e) described earlier schematically show, respectively, aflyback power supply, its operating phases when the switch is closed andthen open, and its waveforms;

FIGS. 2 a) to 2 f) described earlier schematically show, respectively, aforward power supply, its operating phases when the switch is closed andthen open, then when the diode D_(dem) is also open, and its waveforms;

FIGS. 3 a) and 3 b) described earlier are circuit diagrams of,respectively, a flyback power supply and a forward power supply, withleakage inductance;

FIG. 4 described earlier is a circuit diagram of a flyback power supplywith a dissipative RCD circuit;

FIG. 5 described earlier is a circuit diagram of a flyback power supplywith a snubber circuit that dissipates very little energy;

FIG. 6 is a circuit diagram of a unitary coupler according to theinvention;

FIGS. 7 a) and 7 b) are circuit diagrams of, respectively, a flybackpower supply and a forward power supply, with a unitary coupleraccording to the invention;

FIG. 8 is a circuit diagram of an example data transmission system witha unitary coupler according to the invention.

DETAILED DESCRIPTION

The circuit used to reduce, in a power supply, overvoltages across theterminals of the switch M, switching-related losses and losses of energystored in the leakage inductance is a unitary coupler.

A unitary coupler is a transformer in which the winding of the inductorLp of the primary circuit has the same number of turns and the samephase as the winding of the inductor Ls of the secondary circuit.

This also results in the same voltage existing across the terminals ofthe primary inductor Lp and the secondary inductor Ls and therefore, inaccordance with the basic principle stated earlier, a capacitive linkcan be used between these two inductors to counter the effect of thetransformer's leakage inductance.

This type of unitary coupler according to the invention is shown FIG. 6.A first link (link 1) consisting of a first capacitor C1 connects theends of the windings of the inductors Lp and Ls and a second link (link2) consisting of a second capacitor C2, of the same value as the firstcapacitor C1, connects the other ends of the inductors Lp and Ls.

This type of coupler achieves coupling at frequencies ranging fromrelatively low frequencies (of some tens of kHz) to frequencies of sometens of MHz, at the same time reducing losses.

Thus coupling efficiency is increased despite the use of smaller andtherefore less expensive components.

The capacitor chosen for the capacitive links has, as a preference, verylow parasitic series resistance and inductance. For example, amultilayer ceramic capacitor may be used.

This coupler is advantageously used in flyback or forward power suppliesas shown in FIGS. 7 a) and 7 b).

The capacitive links cancel out the overvoltage across the terminals ofthe switch M as M opens. Therefore, RCD or snubber circuits need not beadded and the switch M need not be overdimensioned in terms of voltage.

Furthermore, the energy stored in the leakage inductance is transferreddirectly to the capacitive links that transfer this energy to thesecondary circuit.

Among the various existing flyback and forward power supplyconfigurations, some generate high common mode voltages at the switchingfrequency. Configurations that minimize the common mode voltages betweenthe primary and secondary circuits are chosen so that the capacitivelinks can be used, i.e. configurations for which the voltage across theterminals of capacitor C1 (respectively C2) does not vary as a functionof the switching frequency.

A flyback power supply that does not generate high common mode voltagesat the switching frequency, and that employs a unitary coupler accordingto the invention is shown in FIG. 7 a).

It comprises a primary circuit P consisting of, in series, a voltagesource V_(in), an inductor Lp and a switch M, and a secondary circuit Sconsisting of, in series, a capacitor C_(out) connected to a loadrepresented here by a resistor R_(load), a rectifier D and an inductorLs.

The coupling circuit between the primary circuit P and the secondarycircuit S comprises a unitary coupler according to the invention; theinductors Lp and Ls are therefore identical and connected by capacitivelinks of equal value.

An experimental flyback power supply employing a unitary coupleraccording to the invention has been produced. For an input voltageV_(in)=28 V DC and a power P=50 W, an efficiency gain of about 2 to 5%was achieved with a lowering of overvoltages upon switch-opening by aratio of 4.

A forward power supply that does not generate high common mode voltagesat the switching frequency, and that employs a unitary coupler accordingto the invention is shown in FIG. 7 b).

It comprises a primary circuit P consisting of, in series, a voltagesource V_(in), an inductor Lp and a switch M, and, in parallel with theinductor Lp and the switch M, means for demagnetizing the transformer,for example as per FIG. 2 a). It also comprises a secondary circuit Sconsisting of, in series, a capacitor C_(out) connected to a loadrepresented here by a resistor R_(load), an inductor L, a firstrectifier D1 and an inductor Ls, and, in parallel with the rectifier D1and the inductor Ls, a rectifier D2.

The coupling circuit between the primary circuit P and the secondarycircuit S comprises a unitary coupler according to the invention; theinductors Lp and Ls are therefore identical and connected by capacitivelinks of equal value.

The unitary coupler according to the invention can be applied inparticular to power supplies in which a MOS transistor is used for theswitch M, and/or in which uncontrolled rectifiers such as diodes, oreven controlled rectifiers such as MOS transistors, are used for therectifiers D1, D2 and/or D_(dem).

The unitary coupler according to the invention can in particular beapplied to inductive storage converters such as the ones described inU.S. Pat. Nos. 2,729,471, 2,729,516 and 2,773,013.

The power supplies described achieve improved efficiency and a loweringof the overvoltages across the terminals of the switch, without asignificant increase in the complexity of the circuits as would be thecase for circuits that include RCD or snubber circuits.

Using capacitors means that high-frequency coupling can be achieved, atfrequencies beyond 100 MHz. The unitary coupler according to theinvention may hence be used to transmit data at high frequency: thecapacitive coupling means that a high-speed data transmission ispossible, which is relayed by the magnetic coupling at frequenciesranging from some tens of kHz to several tens of MHz.

An example data transmission system is shown in FIG. 8. It is made up oftwo modules E1 and E2 interconnected via a two-wire data bus B. Eachmodule E1 and E2 has a data transmit-receive device T/R connected to thedata bus B by means of a unitary coupler according to the invention andresistors R.

More generally, the coupler according to the invention may be applied toany device employing a magnetic transformer.

1. A unitary coupler, the coupler comprising: including a first inductorhaving a first winding of phase φ and having a number N of turns betweenthe two ends of the first winding; and, a second inductor, magneticallycoupled to the first inductor, having a second winding of the same phaseφ and having the same number N of turns between the two ends of thesecond winding, wherein the ends of the first and second windings areinterconnected using links consisting of capacitors of equal value.
 2. Aswitch mode power supply having a primary circuit coupled to a secondarycircuit by means of a coupling circuit comprising: a unitary coupler asclaimed in claim
 1. 3. The switch mode power supply as claimed in claim2, wherein the power supply is a flyback type power supply.
 4. Theswitch mode power supply as claimed in claim 3, wherein the primarycircuit includes at least one switch placed in series with a voltagesource and the first inductor, and the secondary circuit includes atleast one rectifier placed in series with the second inductor and acapacitor connected to a load.
 5. The switch mode power supply asclaimed in claim 2, wherein the power supply is a forward type powersupply.
 6. The switch mode power supply as claimed in claim 5, whereinthe primary circuit includes at least one switch placed in series withthe first inductor and a voltage source and, in parallel with the switchand the first inductor, a demagnetizing circuit for demagnetizing themagnetic transformer, and the secondary circuit additionally includes,in series with the second inductor, a capacitor connected to a load, athird inductor, a first rectifier, and, in parallel with the secondinductor and the first rectifier, a second rectifier.
 7. The switch modepower supply as claimed in claim 2 wherein the primary circuit and thesecondary circuit are able to generate at the terminals of eachcapacitor of the unitary coupler a voltage that does not change as afunction of the switching frequency.
 8. The switch mode power supply asclaimed in claim 3 wherein the primary circuit and the secondary circuitare able to generate at the terminals of each capacitor of the unitarycoupler a voltage that does not change as a function of the switchingfrequency.
 9. The switch mode power supply as claimed in claim 5 whereinthe primary circuit and the secondary circuit are able to generate atthe terminals of each capacitor of the unitary coupler a voltage thatdoes not change as a function of the switching frequency.
 10. Datatransmission equipment including at least one data transmit-receivedevice connected to a two-wire data bus, including a unitary coupler asclaimed in claim 1, able to connect the data transmit-receive device tothe two-wire data bus.
 11. The coupler as claimed in claim 1, whereinthe capacitors have very low parasitic series resistance and inductance.12. The coupler as claimed in claim 1, wherein the capacitors aremultilayer ceramic capacitors.
 13. The coupler as claimed in claim 4,wherein the switch is a MOS transistor.
 14. The coupler as claimed inclaim 6, wherein the switch is a MOS transistor.
 15. The coupler asclaimed in claim 4, wherein the rectifier is at least one of a diode anda MOS transistor.
 16. The coupler as claimed in claim 6, wherein atleast one of the first and second rectifier is at least one of a diodeand a MOS transistor.
 17. The coupler as claimed in claim 1, whereincoupling occurs greater than or equal to 100 MHz.